Inorganic Nanoparticle Induced Morphological Transition for Confined

Aug 11, 2017 - ... three-dimensional (3D) confined self-assembly of block copolymers (BCPs) creates the unique nanostructured hybrid composites, which...
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Inorganic Nanoparticle Induced Morphological Transition for Confined Self-Assembly of Block Copolymers within Emulsion Droplets Yan Zhang,†,‡ Yun He,† Nan Yan,*,† Yutian Zhu,*,† and Yuexin Hu‡ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ School of Chemistry and Materials Science, Liaoning Shihua University, Fushun 113001, Liaoning, China S Supporting Information *

ABSTRACT: Recently, it has been reported that the incorporation of functional inorganic nanoparticles (NPs) into the three-dimensional (3D) confined self-assembly of block copolymers (BCPs) creates the unique nanostructured hybrid composites, which can not only introduce new functions to BCPs but also induce some interesting morphological transitions of BCPs. In the current study, we systematically investigate the cooperative self-assembly of a series of size-controlled and surface chemistry-tunable gold nanoparticles (AuNPs) and polystyrene-b-poly(2vinylpyridine) (PS-b-P2VP) diblock copolymer within the emulsion droplets. The influences of the size, content, and surface chemistry of the AuNPs on the coassembled nanostructures as well as the spatial distribution of AuNPs in the hybrid particles are examined. It is found that the size and content of the AuNPs are related to the entropic interaction, while the surface chemistry of AuNPs is related to the enthalpic interaction, which can be utilized to tailor the self-assembled morphologies of block copolymer confined in the emulsion droplets. As the content of PS-coated AuNPs increases, the morphology of the resulting AuNPs/PS-b-P2VP hybrid particles changes from the pupa-like particles to the bud-like particles and then to the onion-like particles. However, a unique morphological transition from the pupa-like particles to the mushroom-like particles is observed as the content of P4VP-coated AuNPs increases. More interestingly, it is observed that the large AuNPs are expelled to the surface of the BCP particles to reduce the loss in the conformational entropy of the block segment, which can arrange into the strings of necklaces on the surfaces of the hybrid particles.

1. INTRODUCTION The self-assembly of block copolymers (BCPs) has attracted intensive attention in recent years since it can generate the nanomaterials with well-ordered nanostructures, which have versatile applications in various fields such as catalyst carrier, electronics, drug delivery, photonics, and so forth.1−6 Clearly, the properties as well as the applications of the self-assembled BCP nanomaterials depend not only on the properties of the BCPs but also on the internal nanostructures and the overall shapes.1,5,7,8 Recently, it was reported that BCPs can selfassemble into a large amount of unique nanostructures under confinement due to the confinement effect and interfacial properties imposed by the confined boundary.7−14 According to the dimension of the confined space, the confinements can be divided into one-dimensional (1D) slit confinement, twodimensional (2D) cylindrical confinement, and three-dimensional (3D) spherical confinement.11−13 Compared to 1D and 2D confinements, 3D confinement is considered as a more powerful implement to break the symmetry of a structure, thus engineering novel nanostructures that are not available in bulk state, or under 1D and 2D confinements.13,15 In experiment, © 2017 American Chemical Society

3D spherical confinement can be achieved via the self-organized precipitation (SORP) method or the emulsion solvent evaporation method.7,15,16 For the emulsion solvent evaporation method, BCPs are encapsulated within a small emulsion droplet with the flexible interface, which can fabricate nanostructured BCP particles with the tunable external shapes, internal phase-separated structures, and surface properties, depending on the intrinsic properties of the BCPs, the water/ oil interfacial properties, and the degree of the confinement.7,13,15,17−22 For instance, Zhu and co-workers proposed a robust strategy of using the mixed surfactants with the tunable composition to manipulate the 3D confined self-assembly of polystyrene-b-polyisoprene-b-poly(2-vinylpyridine) (PS-b-PI-bP2VP), which generated BCP particles with the tunable external shape and internal phase-separated nanostructures.13 Kim and co-workers developed a convenient and effective method to fabricate colloidal patchy particles with various 3D Received: July 8, 2017 Revised: August 9, 2017 Published: August 11, 2017 8417

DOI: 10.1021/acs.jpcb.7b06701 J. Phys. Chem. B 2017, 121, 8417−8425

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by BCP chains and the enthalpic interaction between AuNPs and BCP domain. By tuning the size, content, and surface chemistry of the AuNPs, some unique hybrid BCP particles with well-defined nanostructures are created.

shapes via the confined self-assembly of polystyrene-b-poly(4vinylpyridine) (PS-b-P4VP) block copolymer within the emulsion droplets.15 They found that the shape of the patchy nanoparticles depended on the particle volume and the interfacial interaction at the particle/water interface. On the other hand, a variety of inorganic nanoparticles (NPs) were usually introduced into the emulsion droplets to coassemble with BCPs to create the novel multifunctional hybrid nanomaterials, which can effectively combine the respective advantages of BCPs and NPs.23−29 It is known that the integral properties of the hybrid nanoparticles are highly related to not only the intrinsic properties of BCPs and NPs but also the nanostructures of the hybrid nanomaterials as well as the spatial distribution of NPs within the BCP matrix.26,30−32 Therefore, a great effort has been made in designing the well-defined hybrid nanomaterials with controllable distribution and alignment of NPs.24,25,32−35 In our previous work, we created various hybrid nanostructures via the cooperative self-assembly of PS-b-P4VP block copolymers and gold nanoparticles (AuNPs) confined within the emulsion droplets.33 It was found that the entropic interaction between the AuNPs and BCP domain played an important role in the spatial arrangements of AuNPs as well as the nanostructures of the hybrid particles. Recently, it was reported that the addition of inorganic NPs into BCP domain could not only introduce the functionality to the hybrid nanomaterials but also induce some unique morphological transitions.36−41 For instance, Jang et al. found that the addition of Au-based surfactant nanoparticles into the confined self-assembly of polystyreneb-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer within the emulsion droplets caused a dramatic change in the external shape and internal nanostructure of the resulting BCP particles because these Au-based surfactant nanoparticles were adsorbed at the interface between the block copolymer-containing emulsion droplets and the surrounding amphiphilic surfactant and thus modified the interfacial interactions between the BCP domains and the oil/water interface.24 Recently, Kim and coworkers observed a dramatic transition of both the external shape and internal structure of the PS-b-P4VP particles induced by the addition of size-controlled Au nanoparticle surfactants.25 The AuNPs were preferentially localized at the interface between the P4VP domain at the particle surface and the surrounding water, which can well balance the interfacial interactions between PS/P4VP domains of the BCP particles and water, thus resulting in the morphological transition from the conventional spherical particles to the unique convex lensshaped particles. Although some interesting morphological transitions have been reported before, there is still lack of fundamental understanding on the dependence of the internal structure and the overall shape of the self-assembled BCP particles on the size, content, and surface chemistry of the incorporated NPs. Herein, we synthesize a series of size-controlled and surface chemistry-tunable AuNPs to coassemble with PS-b-P2VP diblock copolymer confined in emulsion droplets. The influences of the size, content, and surface chemistry of the AuNPs on the BCP nanostructures as well as the spatial distribution of AuNPs on BCP domain are investigated. Some interesting morphological transitions of the PS-b-P2VP particles as a function of the AuNP content are observed. Besides the AuNP content, it is found that the hybrid nanostructures also depend on the size and surface chemistry of AuNPs, which is relative to the entropic repulsion imposed

2. EXPERIMENT 2.1. Materials. PS57k-b-P2VP57k (PDI = 1.05), PS2k-SH (PDI = 1.15), P2VP2.5k-SH (PDI = 1.16), and P4VP2.5k-SH (PDI = 1.09) were purchased from Polymer Source, Inc., Canada. Hydrogen tetrachloroaurate trihydrate (HAuCl4· 3H2O, purity 99.99%) and tert-butylamine−borane complex (TBAB) were purchased from Alfa Aesar. Sodium borohydride (NaBH4) and trisodium citrate were obtained from Sinopharm Chemical Reagent. Cetyltrimethylammonium bromide (CTAB, purity 98%), tetraoctylammonium bromide (TOAB), and oleylamine were supplied by Aladdin. Poly(vinyl alcohol) (PVA, average Mw = 13K−23K g/mol, 87−89% hydrolyzed) was obtained from Aldrich. Other chemicals were purchased from Beijing Chemical Factory. All of the materials were used without further purification. The glassware used for synthesizing the gold nanoparticles were cleaned by aqua regia and rinsed with deionized water for a dozen times. 2.2. Synthesis of Ligand-Coated AuNPs. 2.2.1. Synthesis of PS-Coated AuNPs. The PS2k-coated AuNPs (Au1.7S and Au3.5S, 1.7 and 3.5 nm are the diameters of the cores of AuNPs; S represents PS2k-SH ligands) were synthesized by different methods.33,42,43 For the Au1.7S nanoparticles, the Brust twophase method was applied.33,42 First, HAuCl4·3H2O (0.3 mL, 30 mM) aqueous solution and TOAB (0.8 mL, 50 mM) solution in toluene were mixed together. The HAuCl4 was gradually transferred into the toluene phase upon the vigorous stirring, and PS2k-SH (18 mg, 9 μmol) in 0.5 mL of toluene was added to the organic phase. Then, a freshly prepared ice-cold NaBH4 (0.25 mL, 0.4 mol/L) aqueous solution as the reducing agent was dropwise added into the above mixture solution upon the vigorous stirring. The mixed solution was constantly stirred for 3 h. Subsequently, the aqueous layer was removed from the resulting solution, and the Au1.7S NPs were precipitated by adding ethanol and followed by the centrifugation (10 000 rpm, 30 min) to remove the residual TOAB and the unbound PS2k-SH. The resulting AuNPs were then dissolved in chloroform, and a certain amount of ethanol was added to the chloroform solution to induce the precipitation of AuNPs from the solution. The mixture solution was kept overnight at −20 °C. Afterward, the AuNPs were separated by the centrifugation process for 3−5 times to make sure that the unbound PS2k-SH was completely removed. Finally, the purified Au1.7S NPs were redispersed in chloroform and stored at −20 °C. The PS2k-coated AuNPs with the core diameter of 3.5 nm were synthesized by a two-step ligand-exchange approach.33,43 In this case, the citrate-stabilized AuNPs with the diameter of 3.5 nm were first synthesized and then used as the starting materials for the fabrication of Au3.5S NPs. For synthesizing the citrate-stabilized nanoparticles, 20 mL aqueous solution consisting of 2.5 × 10−4 M HAuCl4·3H2O and 2.5 × 10−4 M trisodium citrate was first prepared in a vial. Afterward, 0.6 mL of freshly prepared ice-cold NaBH4 (0.1 M) aqueous solution was quickly injected into the mixture under gentle stirring. The color of the solution immediately turned into pink, indicating the formation of AuNPs. The stirring was kept for 3 min to obtain the citrate-stabilized AuNPs with the diameter of 3.5 nm. These citrate-stabilized AuNPs were used for the following 8418

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(mole ratio Au:SH = 1:0.3). The mixture was sonicated for 2 h and incubated for another 24 h. The following purification process was analogous to that of the Au3.5S. Finally, the Au3.52V was dispersed in chloroform and stored at −20 °C. For Au3.54V, the synthetic procedure was similar to that of Au3.52V, except replacing the P2VP2.5k-SH with P4VP2.5k-SH. 2.2.3. Coassembly of AuNPs and BCPs Confined in Emulsion Droplets. The hybrid AuNPs/BCPs nanoparticles were fabricated by the emulsion solvent evaporation approach. Taking the Au1.7S/PS57k-b-P2VP57k system, for example, a desired amount of Au1.7S NPs was injected into a vial and dried under N2 flow. Then, 0.5 mL of PS57k-b-P2VP57k chloroform solution (5 mg/mL) was added into the vial. The solution was slowly stirred to reach a homogeneous state. Subsequently, 0.1 mL of the mixture solution containing Au1.7S NPs and PS57k-bP2VP57k was emulsified by ultrasonication in 1.0 mL of deionized water containing surfactants (12 mg/mL). The surfactants were consisted of PVA and CTAB (the weight ratio of PVA to CTAB is 3:1). Then, the emulsion solution was stirred at 100 rpm for 4 h at 25 °C under the saturated chloroform vapor atmosphere. Afterward, the chloroform was evaporated slowly in the atmosphere condition with gentle stirring at room temperature for overnight. After the chloroform was completely evaporated, the resulting Au1.7S/PS57k-bP2VP57k hybrid nanoparticles were collected by centrifugation (10 000 rpm, 30 min) to remove the surfactants and then redispersed in deionized water to prepare the transmission electron microscopy (TEM) samples. 2.2.4. Disassembling the Pupa-like AuNPs/BCPs Particles into Nanodiscs. After centrifugation (10 000 rpm, 30 min), the pupa-like Au1.7S/PS57k-b-P2VP57k or Au3.5S/PS57k-b-PS57k particles were dispersed in ethanol, which can selectively disassemble the P2VP layers. The dispersion was initially sonicated for 1 min to promote the disassembly of the P2VP layers. After incubated for 6 h at 30 °C, the solution was sonicated for another 2 min before preparing the TEM samples. 2.3. Characterization. The neat PS-b-P2VP particles and the hybrid AuNPs/PS-b-P2VP particles were visualized with a JEM-1011 instrument operated at 100 kV accelerating voltage. The TEM samples were prepared by dropping 10 μL of the prepared sample suspensions onto a carbon-coated TEM copper grid. The sample was then air-dried at room temperature. To visualize the PS and P2VP domains of AuNPs/PS-b-P2VP hybrid nanoparticles, the sample was exposed to iodine vapor (I2) to selectively stain the P2VP domains for 20 min at 60 °C. The scanning transmission electron microscopy (STEM) image was obtained with a TECNAI G2 high-resolution transmission electron microscope operated at 200 kV. The weight fraction of the surface ligands on AuNPs was measured by TGA (TA Co., Q50). The ligandcoated AuNPs powders were analyzed over temperature range from room temperature to 800 °C at the rate of 10 °C/min under N2 flow.

ligand-exchange process to fabricate Au3.5S NPs within 2−5 h. Specifically, the synthesized citrate-stabilized AuNPs aqueous solution (200 mL) was mixed with the equal volume of PS2kSH (mole ratio Au:SH = 1:0.3) tetrahydrofuran (THF) solution. The mixture solution was sonicated for 2 h, and then incubated for another 24 h. The Au3.5S NPs were collected from the solution by centrifugation (10 000 rpm, 30 min) and redispersed in chloroform. In addition, PS2k-SH chloroform solution was added into the AuNPs chloroform solution again (mole ratio Au:SH = 1:0.15) and followed by the same sonication, incubation, and centrifugation process to improve the graft density of AuNPs. For purifying the Au3.5S NPs, a certain amount of ethanol was added to the chloroform solution to precipitate the Au3.5S from the solution. The mixture solution was kept overnight at −20 °C. Afterward, the same purification procedure was employed to obtain the Au3.5S for 3−5 times. Finally, the resulting Au3.5S NPs were dispersed in chloroform and stored at −20 °C. 2.2.2. Synthesis of P2VP-Coated and P4VP-Coated AuNPs. The P2VP2.5k-coated AuNPs (Au2.02V and Au3.52V, 2.0 and 3.5 nm are the diameters of the cores of AuNPs; 2V represents P2VP2.5k-SH ligands) and P4VP2.5k-coated AuNPs (Au2.04V and Au3.54V; 4V represents P4VP2.5k-SH ligands) were synthesized by the different routes.33,43,44 First, the Au2.02V and Au2.04V with the diameter of 2.0 nm were synthesized by a facile organic phase synthetic approach and a following ligand-exchange process.44 Typically, an orange precursor solution of 1,2,3,4tetrahydronaphthalene (tetralin, 5 mL), oleylamine (5 mL), and HAuCl4·3H2O (50 mg) was mixed. Then, the mixture was sonicated to form a homogeneous solution at room temperature and magnetically stirred under nitrogen (N2) flow for 20 min. Subsequently, the solution was heated to 40 °C for 30 min to reach a stable temperature. Afterward, a reducing solution consisting of 0.25 mmol of TBAB, oleylamine (0.5 mL), and tetralin (0.5 mL) was prepared by ultrasonication and then injected into the above prepared mixture solution. The reaction instantaneously occurred, and the color of the solution changed to a deep purple within 5 s. The reaction continued for 1 h at 40 °C, and a certain amount of acetone was injected to precipitate the AuNPs from the above solution. The AuNPs were separated from the solution by centrifugation (10 000 rpm, 30 min), washed by acetone (three times), and then redispersed in chloroform. To fabricate Au2.02V NPs, the chloroform solution containing P2VP2.5k-SH (320 mg, 128 μmol) ligands was mixed with the above prepared AuNP chloroform solution. The mixture solution was sonicated for 2 h to improve the covalent binding between the AuNPs and P2VP2.5k-SH. The chloroform was then removed by the rotary evaporator, and the AuNPs were redispersed in THF. In order to obtain the purified Au2.02V, a certain amount of deionized water was slowly added into the THF solution to induce the precipitation of AuNPs. The following purification process was in analogy with that of the Au3.5S particles. Finally, the Au2.02V was redispersed in chloroform and stored at −20 °C. For Au2.04V, the synthetic procedure was similar to that of Au2.02V, except replacing the P2VP2.5k-SH with P4VP2.5k-SH. The synthesis of P2VP2.5k-coated AuNPs with the core diameter of 3.5 nm was also achieved by the two-step ligandexchange approach.33,43 First, the citrate-stabilized AuNPs with the diameter of 3.5 nm was synthesized according to the above experimental procedure and used as the starting materials. Then, the citrate-stabilized AuNP solution (100 mL) was added into the equal volume of THF solution containing P2VP2.5k-SH

3. RESULTS AND DISCUSSION The AuNPs/PS57k-b-P2VP57k hybrid particles are generated by the emulsion solvent evaporation method. The surfactants used here are the mixture of PVA and CTAB with a PVA:CTAB weight ratio of 3:1. Clearly, PVA with hydroxyl as the pendant group has a preference to P2VP block while CTAB with a long alkyl tail is affinitive to PS block.13 When PVA:CTAB weight ratio is 3:1, it provides a neutral oil/water interface for both PS and P2VP, which neutralizes the interfacial interaction between 8419

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Figure 1. TEM images of Au1.7S/PS57k-b-P2VP57k hybrid nanoparticles incorporated with different volume fractions of Au1.7S from the 3D confined coassembly within the emulsion droplets: (a) 18.25 vol % Au1.7S, pupa-like particles; (b) 28.65 vol % Au1.7S, bud-shape particles; (c) 36.50 vol % Au1.7S, onion-like particles. The insets in the upper right are the corresponding TEM images for the samples after staining the P2VP domains by I2 vapor, while the insets in the lower right are the corresponding magnified TEM images.

Figure 2. TEM images of Au3.5S/PS57k-b-P2VP57k hybrid nanoparticles incorporated with different volume fractions of Au3.5S from the 3D confined coassembly within the emulsion droplets: (a) 20.44 vol %; (b) 33.95 vol %; (c) 50.69 vol %. The insets in the upper right are the corresponding TEM images for the samples after staining the P2VP domains by I2 vapor, while the insets in the lower right are the corresponding magnified TEM images.

morphological transition of the self-assembled PS57k-b-P2VP57k particles induced by the incorporation of different contents of Au1.7S NPs. When a small amount of Au1.7S NPs (18.25 vol %, the volume fraction of Au1.7S to the sum of Au1.7S and PS blocks) is added to cooperatively self-assemble with the symmetric PS57k-b-P2VP57k block copolymer within the emulsion droplets with a nearly neutral oil/water interface to the PS and P2VP blocks, it is observed that PS57k-b-P2VP57k diblock copolymer aggregate into the ellipsoidal pupa-like particles, while the Au1.7S NPs uniformly locate within the PS lamellas, as shown in Figure 1a. The inset in Figure 1a presents a corresponding TEM image of the sample after selectively staining the P2VP lamellas by I2 vapor, which further confirms that the Au1.7S NPs selectively distribute in the PS lamellas. As the incorporated Au1.7S content is increased to 28.65 vol %, PS57k-b-P2VP57k diblock copolymer tends to form some budlike particles, as shown in Figure 1b. It is found that most of the Au1.7S NPs are enriched at the base of the bud-like particles. When the Au1.7S content is further increased to 36.50 vol %, the classic onion-like particles are generated via the coassembly of Au1.7S and PS57k-b-P2VP57k diblock copolymer (Figure 1c). In the previous study, Jang et al. observed a morphological change from onion-like particles to pupa-like particles induced by the addition of Au-based surfactant nanoparticles.24 Herein, the inverse morphological transition from pupa-like particles to onion-like particles is achieved by increasing the content of Au1.7S NPs. In our current study, the entropic repulsion between the introduced Au1.7S and the PS blocks is significantly increased as the volume fraction of the Au1.7S increases, leading to the flexible PS-coated AuNPs migrating to the oil/water

each block of a PS-b-P2VP block copolymer and the surrounding interface of an emulsion droplet.13 At this PVA:CTAB weight ratio, PS and P2VP blocks aggregate into the axially stacked lamellar nanostructure, and the shape of the particles changes from sphere to ellipsoid due to the commensurability effects associated with the preferred lamellar spacing of the confined block copolymer droplet.24 Therefore, the classic pupa-like particles with the alternated P2VP and PS lamellas are obtained from the confined self-assembly of the neat symmetric PS57k-b-P2VP57k diblock copolymer, as shown in Figure S1. It is calculated that the average thickness of PS and P2VP lamellae are ca. 26.7 and 20.8 nm, respectively. Afterward, a series of different AuNPs (i.e., Au1.7S, Au3.5S, Au2.02V, Au3.52V, Au2.04V, and Au3.54V) with controlled size and surface chemistry are incorporated into the PS57k-bP2VP57k diblock copolymer to engineer the hybrid particles via the emulsion solvent evaporation method. The detailed information and corresponding TEM images of the above AuNPs are shown in Table S1 and Figure S2 of the Supporting Information. It has been reported that both the nanostructures of the hybrid particles and the spatial distribution of NPs in BCP domains are influenced by the entropic and enthalpic interactions.25,27,31−34,40,45−47 The enthalpic interaction is related to the compatibility between the incorporated NPs and the BCP domains, while the entropic interaction is arisen by the conformational loss of BCP chains after the incorporation of solid NPs into BCP domains. Herein, the enthalpic interaction is tailored by the ligands of NPs, while the entropic contributions are controlled by the NP size and NP content. Figure 1 presents a series of TEM images to show the 8420

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The Journal of Physical Chemistry B interface during the evaporation of the organic phase although the oil/water interface is neutral to PS and P2VP. On the other hand, the PS-coated AuNPs have affinity to the CTAB surfactant, which also promotes the migration of the flexible Au1.7S to the surface of the hybrid nanoparticles. Since the AuNPs are coated with PS ligands, it is clear that Au1.7S NPs are affinitive to the PS blocks. Therefore, only the PS blocks are attracted to the interface to form the outermost PS layer with PS-coated AuNPs, thus resulting in the morphological transition from the pupa-like parties to the bud-like particles and then to the onion-like particles as the increase of Au1.7S NPs volume fraction. As a result, most of the Au1.7S NPs are observed at the base of the bud-like particles or the outermost PS layer of the onion-like particles, as shown in the magnified TEM images (the insets of Figures 1b and 1c). In comparison, Au3.5S NPs with larger Au core are also synthesized to coassemble with PS57k-b-P2VP57k diblock copolymer confined in the emulsion droplets. Figure 2 shows a series of TEM images of the resulting Au3.5S/PS57k-b-P2VP57k hybrid particles with different Au3.5S contents. At a relative low Au3.5S content (20.44 vol %), the Au3.5S NPs and PS57k-bP2VP57k diblock copolymer form the ellipsoidal pupa-like particles, as shown in Figure 2a. It is observed that Au3.5S NPs are enriched in the PS lamellae as well as the two PS terminals. With the increase of the Au3.5S content from 20.44 to 33.95 vol %, the resulting hybrid particles change from the ellipsoidal pupa-like structures to the bud-like structures, as shown in Figure 2b. Similar to Figure 1b, most of the Au3.5S NPs are located at the base of the bud-like particles (magnified TEM image inserted in Figure 2b). As the Au3.5S content is further increased to 50.69 vol %, the classic onion-like nanostructures are observed, as shown in Figure 2c. Similar to the morphological transition shown in Figure 1, the morphological transition controlled by the increasing content of Au3.5S is also because that the flexible PS-coated AuNPs are easier to migrate to oil/water interface, which is attributed to the entropic repulsion and the affinity between the PS ligands on AuNPs and CTAB surfactant. More interestingly, some circular Au3.5S stripes are observed at the surface of the onion-like particles, as shown in Figure 2c. Clearly, a large number of flexible Au3.5S NPs are distributed at the surface of the onion-like particles because of the entropic repulsion and the affinity of CTAB at the oil/water interface. As the organic solvent is gradually evaporated from the BCP particles, the flexible Au3.5S NPs may arrange into the cycles because of the coffee ring effect.48−50 To further explore the spatial location of PS-coated AuNPs with different diameters in the PS lamellae, the pupa-like Au1.7S/PS57k-b-P2VP57k and Au3.5S/PS57k-b-P2VP57k particles are selectively disassembled into a series of hybrid PS nanodiscs by ethanol since ethanol is a good solvent for P2VP blocks but a nonsolvent for PS blocks. Figure 3 shows the TEM images and the corresponding STEM images of the disassembled PS nanodiscs from the pupa-like Au1.7S/PS57k-b-P2VP57k and Au3.5S/PS57k-b-P2VP57k particles, respectively. For the hybrid PS nanodiscs disassembled from Au1.7 S/PS57k-b-P2VP 57k (Figure 3a,b), it is observed that the Au1.7S NPs are uniformly distributed in the entire PS discs, i.e., the PS lamellae of the pupa-like Au1.7S/PS57k-b-P2VP57k particles. This implies that the small Au1.7S NPs can be well incorporated into the PS domains because of the relatively weak entropic repulsion imposed by the loss in the conformational entropy of the PS blocks. However, it is interesting to note that all the Au3.5S NPs are located at the edges of PS discs, which arrange into the

Figure 3. (a, c) TEM images of the hybrid nanodiscs disassembled from the pupa-like Au1.7S/PS57k-b-P2VP57k and Au3.5S/PS57k-bP2VP57k particles by ethanol, respectively. (b, d) The corresponding STEM images of (a) and (c), respectively.

strings of necklaces (Figure 3c,d). This is because there is strong entropic penalty when the relatively large Au3.5S NPs are incorporated inside PS domains, resulting in the strong repulsion of Au3.5S NPs to the PS/water interface, i.e., the edge of the PS discs. Xu et al. reported that when the NP size was larger than the interstitial sites between BCP cylinders, the NPs were preferably expelled to the surface of the thin film to avoid the strong entropic penalty.32 In our previous study, it was also observed that the small NPs could well distribute in their affinitive domains, while the large NPs tended to aggregate together at the interface of their affinitive domains because of the strong entropic replusion.33 To quantitatively analyze the entropic effect of the NPs with different diameters, D/L, i.e., the relative size between particle diameter and the preferred domain, is calculated, where D is the total diameter of the AuNP core and the ligand shell and L is the dimension of the selected domain. In the current study, the L value is 26.7 nm, while D values are 6.0 and 10.4 nm for Au1.7S and Au3.5S (see Supporting Information Table S1), respectively. Thus, the D/L values are 0.22 and 0.39 for the hybrid Au1.7S/PS57k-bP2VP57k and Au3.5S/PS57k-b-P2VP57k particles, respectively. Since the D/L value of the Au3.5S/PS57k-b-P2VP57k particles is almost 2 times that of the Au1.7S/PS57k-b-P2VP57k particles, Au3.5S NPs are more likely to be expelled to BCP surface because of a relatively strong entropic penalty. This phenomenon is different from the reported observation in the hybrid thin film of BCP and NPs in the previous works.51,52 In those works, it was found that the relatively large NPs tended to locate at the center of the affinitive domain because the stretching of the affinitive blocks can be reduced by the segregation of the NPs into a central core. Herein, the relatively large are expelled to PS/water interface to reduce the stretching of the affinitive PS blocks. From the above section, it is clear that both the morphology and internal structure of the NPs/BCPs hybrid particles can be tailored by the size and volume fraction of the incorporated NPs. On the other hand, the surface chemistry of NPs is highly relative to the enthalpic interactions between NPs and BCP domains, which also plays an important role in the confined 8421

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Figure 4. TEM images of hybrid Au2.02V/PS57k-b-P2VP57k nanoparticles (a−c) and Au3.52V/PS57k-b-P2VP57k nanoparticles (d−f) incorporating different volume fractions of Au2.02V or Au3.52V from the 3D confined coassembly within the emulsion droplets: (a) 12.84 vol % Au2.02V; (b) 33.38 vol % Au2.02V; (c) 54.10 vol % Au2.02V; (d) 25.87 vol % Au3.52V; (e) 34.92 vol % Au3.52V; (f) 51.77 vol % Au3.52V. The insets in the lower right are the corresponding TEM images for the samples after staining the P2VP domains by I2 vapor, while the insets in the upper right are the corresponding magnified TEM images.

Figure 5. TEM images of the Au2.04V/PS57k-b-P2VP57k hybrid nanoparticles (a−c) and Au3.54V/PS57k-b-P2VP57k hybrid nanoparticles (d−f) incorporating with different volume fractions of Au2.04V or Au3.54V from the 3D confined coassembly within the emulsion droplets: (a) 22.39 vol %; (b) 53.58 vol %; (c) 74.26 vol %; (d) 11.68 vol %; (e) 51.42 vol %; (f) 72.57 vol %. The inset in the upper right is the corresponding TEM images with the P2VP domains stained by I2 vapor.

coassembly of AuNPs and BCPs. Therefore, we synthesized the P2VP2.5k-modified AuNPs (Au2.02V and Au3.52V) and P4VP2.5kmodified AuNPs (Au2.04V and Au3.54V) to coassemble with PS57k-b-P2VP57k block copolymer within the emulsion droplets. Figures 4a−c and 4d−f show the morphological transitions of PS57k-b-P2VP57k particles induced by the incorporation of Au2.02V and Au3.52V, respectively. Since the AuNPs are modified by P2VP2.5k-SH ligands, both Au2.02V and Au3.52V are selectively distributed in their affinitive P2VP domains, as shown in Figure 4. Similar to the hybrid Au1.7S/PS57k-b-

P2VP57k or Au3.5S/PS57k-b-P2VP57k particles, the morphological transition from the pupa-like particles to the bud-like particles and then to the onion-like particles is observed as the increase of Au2.02V content (Figure 4a−c). In addition, the distinct aggregation of Au3.52V NPs on the surfaces of PS57k-b-P2VP57k particles is observed for the coassembly of Au3.52V and PS57k-bP2VP57k. It is worth to note that the Au3.52V NPs tend to aggregate into some circular stripes on the surface of the onionlike particles (Figure 4e) when the Au3.52V content is 34.92 vol %. The formation of these circular stripes may be attributed to 8422

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the coffee ring effect, which is similar to that in Figure 2c.48,50,53 As the Au3.52V content is further increased to 51.77 vol %, the Au3.52V NPs bestrew the whole surfaces of the onion-like PS57kb-P2VP57k particles (Figure 4f). When the AuNPs are modified by P4VP2.5k-SH ligands (Au2.04V or Au3.54V), a completely different morphological transition is observed after the incorporation of the P4VP2.5kmodified AuNPs into the 3D confined self-assembly of PS57k-bP2VP57k diblock copolymer. In Figure 5, we present a series of TEM images of the Au2.04V/PS57k-b-P2VP57k hybrid particles and the Au 3.54V/PS 57k-b-P2VP57k hybrid particles with incorporating different volume fractions of P4VP2.5k-modified AuNPs. It is worth to note that all the P4VP2.5k-modified AuNPs are concentrated at one pole of the pupa-like PS57k-bP2VP57k particles. A similar phenomenon has been observed in our previous study.33 In that work, it was found that the Au3.54V NPs were concentrated at one pole of the pupa-like PS9.8k-b-P4VP10k particles when the Au3.54V and PS9.8k-bP4VP10k block copolymer coassembled within the emulsion droplets. The formation of this unique hybrid nanostructure is attributed to the synergistic effect of multiple factors. First, the entropic repulsion imposed by the polymer chains tends to expel AuNPs from BCP domains to the polymer/solution interface. Second, the oil/water interface is slightly selective to P4VP because the P4VP possesses slight hydrophilicity and relatively strong polarity, which will attract the flexible P4VPmodified AuNPs to the polymer/solution interface.22 Finally, the P4VP-modified AuNPs are more likely to concentrate together at one pole of the pupa-like particles to reduce the contact with the polymer chains when they are at the surface of BCP particle, which can further reduce the entropic penalty. As the AuNPs content is increased, more and more P4VP2.5kmodified AuNPs are enriched on one pole of the particles. More interestingly, it is also observed that the unloaded pole of the particles is gradually disappeared as the increase of AuNPs content, which causes a unique morphological transition from the pupa-like particles to the mushroom-like particles, as shown in Figure 5. Clearly, the gravity center of the hybrid particle will gradually deviate from the geometric center to the pole enriched with AuNPs when more and more AuNPs are incorporated. This may cause the hybrid particles rotate under the shear flow, thus resulting in the disassembly of the unloaded pole.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b06701. Characteristics of the different ligand-coated gold nanoparticles used in our experiments; TEM image of the neat block copolymer nanoparticles fabricated by the 3D confined self-assembly of PS57k-b-P2VP57k block copolymer; TEM images of the AuNPs used in our experiments (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. *E-mail [email protected]. ORCID

Yutian Zhu: 0000-0002-7092-0086 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China for General Program (51373172), Major Program (51433009), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201734).

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REFERENCES

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4. CONCLUSIONS In this work, we investigated the cooperative self-assembly of AuNPs and PS-b-P2VP diblock copolymer confined in the emulsion droplets. The influences of the size, content, and surface chemistry of the incorporated AuNPs on the selfassembled morphologies are systematically examined. As the incorporated AuNP content increases, some interesting morphological transitions of the self-assembled particles, such as the transition from the pupa-like particles to the bud-like particles and then to the onion-like particles and the transition from the pupa-like particles to the mushroom-like particles, are observed, depending on the size and surface chemistry of AuNPs. Moreover, it is also found that these morphological transitions induced by the addition of AuNPs are related to the entropic and enthalpic interactions, which can be tailored by the size, content, and surface chemistry of AuNPs. 8423

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